This major project has been aimed at developing new strategies to understand the basic molecular mechanisms that govern initial recognition events in the immune response. As examples of such molecular recognition events, we have focused on interactions of the major histocompatibility complex (MHC) molecules with specific T cell receptors or of MHC class I molecules with receptors on natural killer cells. We have developed model systems for producing large quantities of the molecules involved: MHC molecules, T cell receptors and their fragments, and NK receptors. After expression and purification, these molecules have been used to examine structure, biophysical parameters of binding, and function. Major accomplishments in the past year include the production of T cell receptor domains of sufficient purity and quantity to allow both crystallization and high resolution nuclear magnetic resonance spectroscopy. The x-ray crystallographic structure of a Valpha domain has been solved, and high resolution refinement is underway. A strategy for production of an active T cell receptor combining site with specificity for a particular MHC/peptide complex has proven effective in staining cell surface MHC/peptide complexes on living cells, and in blocking specific T cell activation. This strategy for producing such biologically active engineered T cell receptors in bacteria has now been applied to additional T cell receptors. Such studies offer new insights into the molecular rules that govern the initiation of the immune response, and also provide a plausible approach to make molecules and ultimately drugs that can specifically inhibit T cells that might run out of control in autoimmune conditions.